1. Field of the Invention
The present invention relates to a laser beam applying mechanism and a laser processing apparatus which can control at high speeds the power of a laser beam oscillated by a laser beam oscillator.
2. Description of the Related Art
In a semiconductor device fabrication process, a plurality of crossing division lines called streets are formed on the front side of a substantially disk-shaped semiconductor wafer to thereby partition a plurality of regions where devices such as ICs and LSIs are respectively formed. The semiconductor wafer is cut along the streets to thereby divide the regions where the devices are formed from each other, thus obtaining the individual semiconductor devices.
Cutting of the semiconductor wafer along the streets is usually performed by using a cutting apparatus called a dicing saw. This cutting apparatus includes a chuck table for holding the semiconductor wafer as a workpiece, cutting means for cutting the semiconductor wafer held on the chuck table, and moving means for relatively moving the chuck table and the cutting means. The cutting means includes a spindle adapted to be rotated at high speeds and a cutting blade mounted on the spindle. The cutting blade is composed of a disk-shaped base and an annular cutting edge mounted on a side surface of the base along the outer circumference thereof. The cutting edge is formed by fixing diamond abrasive grains having a grain size of about 3 μm to the base by electroforming so that the thickness of the cutting edge becomes about 20 μm, for example.
In recent years, a semiconductor wafer intended to improve the processing performance of circuits such as ICs and LSIs has been put into practical use. This semiconductor wafer is composed of a semiconductor substrate such as a silicon wafer and a low-permittivity insulator film (Low-k film) formed on the front side of the semiconductor substrate. Examples of the Low-k film include an inorganic film of SiOF, BSG (SiOB), etc. and an organic film such as a polymer film of polyimide, parylene, etc. However, the Low-k film is formed as a multilayer film (5 to 15 layers) like mica and it is very brittle. Accordingly, when this semiconductor wafer is cut along the streets by using the cutting blade, there arises a problem such that the Low-k film may be separated and this separation may reach the circuits to cause fatal damage to the semiconductor chips. To solve this problem, Japanese Patent Laid-open No. 2003-320466 discloses a processing apparatus for applying a laser beam to the Low-k film along the streets to remove the Low-k film on the streets and next cutting the streets where the Low-k film is now absent by using a cutting blade.
However, in a semiconductor wafer having a configuration such that a plurality of testing metal patterns called test element group (TEG) for testing the function of the circuits are partially provided on the Low-k film in the streets, there is a problem such that the laser beam for removing the Low-k film is blocked by the metal patterns of copper, aluminum, etc., so that the Low-k film cannot be smoothly removed. In the case that the power of the laser beam to be applied to the streets is increased to such an extent that the metal patterns can be removed, there arises a new problem such that the semiconductor substrate forming the streets where only the Low-k film is present may be damaged by the laser beam, causing scattering of debris, and this debris may be deposited to bonding pads or the like connected to the circuits, causing a degradation in quality of the semiconductor chips.
To solve such a problem, Japanese Patent Laid-open No. 2005-118832 discloses a laser processing method of applying a laser beam to an area where the metal patterns are formed and an area where the Low-k film is formed under different processing conditions, thereby removing the metal patterns and the Low-k film.